New results of radiation environment investigation by Liulin-5 experiment in the human phantom aboard the International Space Station

Semkova J. 1, Koleva R. 1, Maltchev S. 1, Benghin V. 2, Chernykh I. 2, Shurshakov V. 2, Petrov V. 2, Bankov N. 3, Goranova M. 4 1) Solar-Terrestrial Influences Institute, Bulgarian Academy of Sciences, 2)Institute of Biomedical Problems, Russian Academy of Sciences 3) Space Research Institute, Bulgarian Academy of Sciences 4) Technical University, Bulgaria

OUTLINE   Introduction   Liulin - 5 instrument of MATROSHKA-R experiment  Some results  Conclusions  Future works

INTRODUCTION  Experiment Liulin-5 for investigation of the radiation environment dynamics within the spherical tissue-equivalent phantom on ISS started in June 2007 on RS of ISS. Since then it runs on ISS.  Liulin-5 experiment is a part of the international project MATROSHKA-R.  We present some results of analysis of the data obtained during July March 2009 at the minimum of solar activity cycle and quiet solar and geomagnetic conditions.

Spherical Tissue – Equivalent Phantom  Size: 370x370x390 mm; mass: 32kg;  Radiation detectors – number of passive detectors and Liulin-5 charged paricle telescope.  Liulin-5 detector module is placed in a radial channel.

Liulin-5 experiment objectives Main objective - to study the depth-dose distribution of the different components of the orbital radiation field in a human phantom. Main objective - to study the depth-dose distribution of the different components of the orbital radiation field in a human phantom. Additional objectives are mapping of the radiation environment in the phantom and its variations with time and orbital parameters (such as solar cycle, solar flare events, inclination and altitude). Additional objectives are mapping of the radiation environment in the phantom and its variations with time and orbital parameters (such as solar cycle, solar flare events, inclination and altitude).

Block - diagram of Liulin - 5 connections in the phantom

LIULIN – 5 EXPERIMENT Goals  Liulin-5 measures simultaneously at 3 different depths of the radial channel of the spherical phantom: Energy Deposition Spectra, Dose Rate & Particle flux - then Absorbed Dose D;  Measurement of the Linear Energy Transfer (LET) spectra in silicon – then assessment of LET(H 2 O), Q=f(LET), given in ICRP-60 and Dose Equivalent H; H=DxQ.

Phantom Detector Module Electronic Block Liulin-5 in the spherical phantom External view of Liulin-5 Two units: a detector module and an electronic block.

Parameters provided   Absorbed dose rate in the range 0.04 x10 -6 Gy/h Gy/h;   Particle flux in the range 0 - 4x10 2 particle/(cm 2.sec);   Energy deposition spectra in 512 spectral channels: In 1-st and 2-nd detectors in the range 0.45 – 63 MeV; in 3-rd detector in the range 0.2 –10 MeV;   LET (Si) spectra  LET(H 2 O) spectra in the range 0.65 –90 keV/µm.   All events exceeding the upper spectral limits are recorded in the 512 channel.

LIULIN-5 in the Phantom in Piers-1 module of ISS –activated 28 June Spherical Phantom LIulin -5 in the Spherical Phantom on ISS Detector module

Position of Piers module at the ISS Piers

Phantom in PIERS module

RESULTS  Absorbed energy spectra, LET spectra.   Dose distribution in the radial channel of the phantom.   Dosemetric quantities from the different components of the radiation environment in ISS.

Energy deposition spectra, dose rates, LET spectrum, and quality factor estimation  Data for  3 upper panels – energy deposition spectra and absorbed dose rates in 3 detectors; Dose rates in the equal for all detectors range 0.65  LET  14 keV/um : D 1av =8.2 uGy/h, D 2av =7.9 uGy/h, D 3av =4.5 uGy/h;  4-th panel - LET spectrum  0.65 keV/um and Q; D LET =13 uGy/h. The value Qav=3.2 is obtained when all events, exceeding the upper LET measurement limit (0.11%) are considered as events with LET 90 keV/µm (corresponding to maximum Q).

Absorbed dose depth distribution  Averaged daily doses for measured at 40, 60 and 165 mm depth.  Fast mode- mainly SAA protons. Dose at 165 mm decreased by 2.7.  Standard mode – mainly GCR. Decreasing by 1.4  Total. The total absorbed dose at 165 mm depth in the phantom is 1.8 times less than that at 40 mm mainly due to self-shielding of the phantom against trapped radiation.

Comparison with measurements of passive detectors Typical depth-dose curve from TLDs along the diameter perpendicular to the space station wall -decreasing by between the doses measured at 40 mm and 165 mm from the phantom’s surface.

Distribution of dose rate in geographic coordinates ( ) Dose rates distribution at 40 and 165 mm depth: D1  565 µGy/h, D3  188 µGy/h. Data from D1 is for 0.65  LET  90keV/µm, from D3 – for 0.3  LET  14keV/µm.

LET spectra ( ) LET measurements in the range keV/µm.  0.16% of all events of the total LET spectrum are in 512 channel - exceed the upper LET limit. Absorbed dose rate [µGy/h]: SAA = 71.4, GCR= 6.0, Total =8.4

Radiation quantities obtained at 40 cm distance from the phantom’s surface calculated from LET spectra, events in the last LET channel ignored 0.65  LET< 90 keV/µm ( ) QuantityQavDaily absorbed dose [µGy/day] Daily Dose equivalent [µSv/day] Value

Radiation quantities from the different components of the radiation field at 40 cm distance from the phantom’s surface ( , calculated from LET spectra, 0.65  LET  90 keV/µm Considering the events in the last spectral channel as events of 90 keV/µm, Qav increases significantly and the dose equivalent of the LET spectrum at 40 cm depth at the phantom increases with about 40%, compared to that with LET<90 keV/  m. Dose equivalent in SAA only ~12% of total dose equivalent, dose equivalent from GCR- ~88%.

Strong dependence of dose rates and particle fluxes in SAA of the shielding and ISS attitude Flux and dose rates in D1 and D2 measured for 0.65  LET  90keV/µm, in D  LET  14keV/µm. Data for: Fluxes in D1&D2 bigger than in D3. ( and after 26.03). During fluxes in SAA decreased more than 2 times due to ISS attitude changing and docking of the SHUTLLE (STS-123).luxes in D3 bigger than in D2 and D Fluxes in D1&D2 bigger than in D3. ( and after 26.03). During fluxes in SAA decreased more than 2 times due to ISS attitude changing and docking of the SHUTLLE (STS-123). Fluxes in D3 bigger than in D2 and D Docking of the Progress-M66 cargo vehicle on Dose rates in D2 decreased twice during

The daily doses of 2 external detectors decreased by factor of 1.3 during the docking of STS-122 and 119. The heavy shielded 3th detector response is very small STS-122 docked D1 D2 D3 STS docked

Comparison of the dose rates in SAA from different instruments on ISS close to STS-123 docking TEPC (NASA- JSFC) – in US Lab. ModuleTEPC (NASA- JSFC) – in US Lab. Module D3DE (STIL- BAS)-outside ESA Columbus ModuleD3DE (STIL- BAS)-outside ESA Columbus Module Liulin-5 (STIL- BAS)– in PIERS of Russian Service ModuleLiulin-5 (STIL- BAS)– in PIERS of Russian Service Module Thanks Dr. E. Semones for the TEPC and Prof. Dachev for R3DE data

CONCLUSIONS (1) ISS orbital parameters.  Data obtained in July, March, 2009 show that the dose rates and fluxes measured in SAA are the most intensive and strongly depend on the shielding of detectors in the phantom and ISS orbital parameters.  During July 2007-April 2008 in SAA the absorbed doses at the center of the phantom are times lower than at mm distance from the surface. The total absorbed dose from all space radiation sources at 165 mm depth in the phantom is times less than that at 40 mm.

CONCLUSIONS (2)   The results are indicative for the GCR heavy charged particles and SAA protons contribution to the average quality factor and dose equivalent in the phantom.   At the minimum of the solar cycle the dose equivalent from GCR is more than 80% of total dose equivalent at the depth of the blood- forming organs. The rest dose equivalent is due to the trapped radiation.   The results of radiation investigations with phantoms in space to be taken into account in planning of future exploratory manned missions.

FUTURE WORKS (1)  Additional analysis for estimating the contribution of the radiation with LET (H2O) < 0.65 keV/  m and with LET (H2O)  90 keV/  m to the LET spectra and radiation quantities of the GCR and trapped particles, including comparisons with models of the radiation environment, shielding conditions for Liulin-5 detectors and with data from other dosemeters of MATROSHKA-R experiment.   The Liulin-5 experiment continues on ISS.

FUTURE WORKS (2)   A new experiment for radiation research with a new charged particle telescope Liulin-F will be flown onboard the Phobos-Soil space mission (Launch expected October 2009). Measurement parameters similar to Liulin-5.   Measurements of radiation conditions during the cruise phase, on Mars’s orbit and on the surface of Phobos are planned. Objectives: Radiation doses received by the components of spacecraft. Verification the radiation environment models and assessment of radiation risk to the crewmembers of future exploratory flights.

ACKNOWLEDGEMENTS  Agreement between RAS and BAS on space research and grant HZ-1505/2005 from the Bulgarian Ministry of Education and Science.  Thanks the cosmonauts O. Kotov, Y. Malenchenko, O. Kononenko and Y. Lonchakov for the operation of Liulin - 5 aboard ISS.

Thank you for attention!